The subject disclosure relates to superconducting quantum circuits, and more specifically, to external port measurement of qubit port responses.
Quantum computing is generally the use of quantum-mechanical phenomena for the purpose of performing computing and information processing functions. Quantum computing can be viewed in contrast to classical computing, which generally operates on binary values with transistors. That is, while classical computers can operate on bit values that are either 0 or 1, quantum computers operate on quantum bits that comprise superpositions of both 0 and 1, can entangle multiple quantum bits, and use interference.
Quantum computing hardware can be different from classical computing hardware. In particular, superconducting quantum circuits generally rely on Josephson junctions, which can be fabricated in a semiconductor device. A Josephson junction generally manifests the Josephson effect of a supercurrent, where current can flow indefinitely across a Josephson junction without an applied voltage. A Josephson junction can be created by weakly coupling two superconductors (a material that conducts electricity without resistance), for example, by a tunnel barrier.
One way in which a Josephson junction can be used in quantum computing is by embedding the Josephson junction in a superconducting circuit to form a quantum bit (qubit). A Josephson junction can be used to form a qubit by arranging the Josephson junction in parallel with a shunting capacitor. A plurality of such qubits can be arranged on a superconducting quantum circuit fabricated on a semiconductor device. The qubits can be arranged in a lattice (e.g., a grid) formation such that they can be coupled to nearest-neighbor qubits. Such an arrangement of qubits coupled to nearest-neighbor qubits can constitute a quantum computing architecture. An example of an existing quantum computing architecture is the quantum surface code architecture, which can further comprise microwave readout resonators coupled to the respective qubits that facilitate reading quantum information of the qubits (also referred to as addressing or reading a quantum logic state of the qubit). Such a quantum surface code architecture can be integrated on a semiconducting device to form an integrated quantum processor that can execute computations and information processing functions that are substantially more complex than can be executed by classical computing devices (e.g., general-purpose computers, special-purpose computers, etc.).
Superconducting quantum devices (e.g., a superconducting quantum chip, a quantum processor, etc.) can comprise internal ports (e.g., qubit ports) that can be defined across terminals of a qubit, for example, across terminals of a Josephson junction. As referenced herein, a qubit port can comprise an internal lumped port that can be defined across terminals of Josephson junctions. The internal ports (e.g., qubit ports) can be probed to determine information such as, for example, port response functions (e.g., impedance matrices, admittance matrices, etc.). Such information obtained from probing qubits (e.g., probing qubit ports) can be utilized, for example, to: construct Hamiltonians of a circuit quantum electrodynamic (circuit-QED) system; determine exchange coupling rates between qubits; and/or determine classical cross-talk measures corresponding to multiple qubits. As referenced herein, a Hamiltonian can comprise an operator that can correspond to the total energy of a system (e.g., a circuit-QED), where the spectrum of a Hamiltonian can comprise a set of possible outcomes that can result from measuring the total energy of the system.
A challenge associated with probing internal ports (e.g., qubit ports of qubits) is the difficulty of accessing such ports, which are internal to a superconducting quantum device (e.g., a superconducting quantum chip, a quantum processor, etc.). Another challenge associated with probing internal ports such as, for example, qubit ports, is the difficulty of physically connecting radio frequency (RF) probes to one or more terminals of a qubit (e.g., Josephson junction terminals), in a non-invasive manner, due to the small size of such terminals.
Existing superconducting quantum systems provide for constructing Hamiltonians from port response functions by defining internal ports (e.g., qubit ports) and external ports. Some prior art systems illustrate how the response of a system seen at the qubit ports is enough to construct the full Hamiltonian of the superconducting device. In such prior art systems, the port response functions (e.g., impedance matrices, admittance matrices, etc.) seen at the qubit ports are assumed to be calculated in Electromagnetic Simulation software systems. A problem associated with such prior art systems is they fail to disclose extracting internal (qubit) port responses from port response functions measured at the external ports, which can be used to obtain and/or validate Quantum Hamiltonian models of a superconducting qubit circuit.